60-419: A penis ( / ˈ p iː n ɪ s / ; pl. : penises or penes ) is a male sex organ that is used to inseminate female or hermaphrodite animals during copulation . Such organs occur in both vertebrates and invertebrates , including humans, but not in all male animals. The term penis applies to many intromittent organs , but not to all. As an example, the intromittent organ of most Cephalopoda
120-463: A crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy. Though traditionally considered reptiliomorphs, some recent research has recovered diadectomorphs as the sister group to Synapsida within Amniota, based on inner ear anatomy. The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows
180-400: A prepuce when not erect. Mammals have either musculocavernous penises, which expand while erect, or fibroelastic penises, which become erect by straightening without expanding. Preputial glands are present in some prepuces. The penis bears the distal part of the urethra in placental mammals . The perineum of testicond mammals (mammals without a scrotum ) separates the anus and
240-493: A calcified shell, were not essential and probably evolved later. It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4-inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit
300-472: A common primordium during early gestation and differentiate into male or female sexes . The SRY gene , usually located on the Y chromosome and encoding the testis determining factor , determines the direction of the differentiation. The absence of it allows the gonads to continue to develop into ovaries. The development of the internal and external reproductive organs is determined by hormones produced by certain fetal gonads (ovaries or testicles) and
360-402: A fetus. The term originally described a bowl in which the blood of sacrificed animals was caught, and derived from ἀμνός ( amnos ), meaning "lamb". Zoologists characterize amniotes in part by embryonic development that includes the formation of several extensive membranes, the amnion , chorion , and allantois . Amniotes develop directly into a (typically) terrestrial form with limbs and
420-403: A few mammals ( monotremes , tenrecs , golden moles , and marsupial moles ) have this orifice, from which they excrete both urine and feces in addition to serving reproductive functions. Excretory systems with analogous purpose in certain invertebrates are also sometimes referred to as cloacae. Penile and clitoral structures are present in some birds and many reptiles. Sexing teleost fish
480-401: A penis. Most male birds (e.g., roosters and turkeys ) have a cloaca (also present on the female), but not a penis. Among bird species with a penis are paleognaths ( tinamous and ratites ) and Anatidae (ducks, geese and swans). The magpie goose in the family Anseranatidae also has a penis. A bird penis is different in structure from mammal penises, being an erectile expansion of
540-471: A preputial sheath in the male's urogenital sinus when not erect. Monotremes and marsupial moles are the only mammals in which the penis is located inside the cloaca. Male turtles and crocodilians have a penis, while male specimens of the reptile order Squamata , which are snakes and lizards , have two paired organs called hemipenes . Tuataras must use their cloacae for reproduction. Due to evolutionary convergence , turtle and mammal penises have
600-411: A primary factor for genital evolution. Male genitalia show traits of divergent evolution that are driven by sexual selection. The visible portion of eutherian mammalian genitals for males consists of the penis and scrotum ; for females, it consists of the vulva . In placental mammals , females have two genital orifices, the vaginal and urethral openings , while males have only one for
660-401: A similar structure. In some fish, the gonopodium, andropodium, and claspers are intromittent organs (to introduce sperm into the female) developed from modified fins. Harvestmen are the only male arachnids that have a penis . In male insects , the structure analogous to a penis is known as an aedeagus . The male copulatory organ of various lower invertebrate animals is often called
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#1732780951838720-434: A thick stratified epithelium (rather than first entering a feeding larval tadpole stage followed by metamorphosis , as amphibians do). In amniotes, the transition from a two-layered periderm to a cornified epithelium is triggered by thyroid hormone during embryonic development, rather than by metamorphosis. The unique embryonic features of amniotes may reflect specializations for eggs to survive drier environments; or
780-612: Is determined by the shape of a fleshy tube behind the anus known as genital papilla . The organs concerned with insect mating and the deposition of eggs are known collectively as the external genitalia, although they may be largely internal; their components are very diverse in form. The reproductive system of gastropods (slugs and snails) varies greatly from one group to another. Planaria are flat worms widely used in biological research. There are sexual and asexual planaria. Sexual planaria are hermaphrodites, possessing both testicles and ovaries. Each planarian transports its excretion to
840-450: Is presented in simplified form below. With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny , but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants". As Gauthier makes use of
900-426: Is produced in stamens and is carried to the pistil or carpel , which has the ovule at its base where fertilization can take place. Within each pollen grain is a male gametophyte, which consists of only three cells. In most flowering plants, the female gametophyte within the ovule consists of only seven cells. Thus there are no sex organs as such. The sex organs in fungi are known as gametangia . In some fungi,
960-688: Is the hectocotylus , a specialized arm, and male spiders use their pedipalps . Even within the Vertebrata , there are morphological variants with specific terminology, such as hemipenes . The word "penis" is taken from the Latin word for " tail ". Some derive that from Indo-European *pesnis , and the Greek word πέος = "penis" from Indo-European *pesos . Prior to the adoption of the Latin word in English,
1020-526: The Bashkirian age of the Late Carboniferous around 318 million years ago . This basal divergence within Amniota has also been dated by molecular studies at 310–329 Ma, or 312–330 Ma, and by a fossilized birth–death process study at 322–340 Ma. The term amniote comes from the amnion , which derives from Greek ἀμνίον ( amnion ), which denoted the membrane that surrounds
1080-602: The bulbus glandis . During copulation, the spotted hyena inserts his penis through the female's pseudo-penis instead of directly through the vagina , which is blocked by the false scrotum. The pseudo-penis and pseudo-scrotum, which are actually a masculinized vulva , closely resemble the male hyena's genitalia, but can be distinguished from the male by the female's greater thickness and more rounded glans . Domestic cats have barbed penises, with about 120–150 one millimetre long backwards-pointing spines . Marsupials usually have bifurcated penises that are retracted into
1140-430: The cirrus . In 2010, entomologist Charles Linehard described a new genus of barkflies called Neotrogla . Species of this genus have sex-reversed genitalia: females have penis-like organs called gynosomes that are inserted into vagina-like openings of males during mating. A similar female structure has also been described in the closely related Afrotrogla . Scientists who study these insects have occasionally called
1200-482: The gametophyte . The flowers of flowering plants produce pollen and egg cells , but the sex organs themselves are inside the gametophytes within the pollen and the ovule. Coniferous plants likewise produce their sexually reproductive structures within the gametophytes contained within the cones and pollen. The cones and pollen are not themselves sexual organs. Together, the sex organs constitute an organism's reproductive system . The primary sex organs are
1260-423: The gonads , a pair of internal sex organs, which diverge into testicles following male development or into ovaries following female development. As primary sex organs, gonads generate reproductive gametes containing inheritable DNA . They also produce most of the primary hormones that affect sexual development, and regulate other sexual organs and sexually differentiated behaviors. Secondary sex organs are
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#17327809518381320-473: The placenta . The ancestors of true amniotes, such as Casineria kiddi , which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from
1380-492: The semiaquatic amphibians do. They have better homeostasis in drier environments, and more efficient non-aquatic gas exchange to power terrestrial locomotion , which is facilitated by their astragalus. Basal amniotes resembled small lizards and evolved from semiaquatic reptiliomorphs during the Carboniferous period. After the Carboniferous rainforest collapse , amniotes spread around Earth's land and became
1440-490: The amnion and the fluid it secretes shields the embryo from environmental fluctuations, amniotes can reproduce on dry land by either laying shelled eggs (reptiles, birds and monotremes ) or nurturing fertilized eggs within the mother ( marsupial and placental mammals ). This distinguishes amniotes from anamniotes ( fish and amphibians) that have to spawn in aquatic environments . Most amniotes still require regular access to drinking water for rehydration, like
1500-449: The amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and
1560-432: The body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock". See WikiSaurus:penis for a list of alternative words for penis. The Latin word " phallus " (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe representations , pictorial or carved, of
1620-399: The cells' response to them. The initial appearance of the fetal genitalia looks female-like: a pair of urogenital folds with a small protuberance in the middle, and the urethra behind the protuberance. If the fetus has testes and the testes produce testosterone, and if the cells of the genitals respond to the testosterone, the outer urogenital folds swell and fuse in the midline to produce
1680-555: The class Reptilia is paraphyletic —it has given rise to two other classes not included in Reptilia. Most species described as microsaurs , formerly grouped in the extinct and prehistoric amphibian group lepospondyls , has been placed in the newer clade Recumbirostra , and shares many anatomical features with amniotes which indicates they were amniotes themselves. A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton ,
1740-401: The cloacal wall (in ducks) and being erected by lymph , not blood. It is usually partially feathered and in some species features spines and brush-like filaments, and in a flaccid state, curls up inside the cloaca. As with any other bodily attribute, the length and girth of the penis can be highly variable between mammals of different species . In many mammals, the penis is retracted into
1800-473: The course of evolution. An erection is the stiffening and rising of the penis, which occurs during sexual arousal , though it can also happen in non-sexual situations. During ejaculation , a series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa , from the penis. Ejaculation is usually accompanied by orgasm . The last common ancestor of all living amniotes (mammals, birds and reptiles) likely possessed
1860-437: The development and birth of offspring. Sex organs are found in many species of animals and plants, with their features varying depending on the species. Sex organs are typically differentiated into male and female types. In animals (including humans), the male sex organs include the testicles , epididymides , and penis ; the female sex organs include the clitoris , ovaries , oviducts , and vagina . The testicle in
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1920-425: The development of three extraembryonic membranes ( amnion for embryonic protection, chorion for gas exchange , and allantois for metabolic waste disposal or storage), thicker and keratinized skin , costal respiration (breathing by expanding/constricting the rib cage ), the presence of adrenocortical and chromaffin tissues as a discrete pair of glands near their kidneys , more complex kidneys ,
1980-402: The diapsid line of descent. Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs , a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle. Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion , hence
2040-438: The dominant land vertebrates. They almost immediately diverged into two groups, namely the sauropsids (including all reptiles and birds ) and synapsids (including mammals and extinct ancestors like " pelycosaurs " and therapsids ). Among the earliest known crown group amniotes, the oldest known sauropsid is Hylonomus and the oldest known synapsid is Asaphestera , both of which are from Nova Scotia during
2100-400: The early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof . In their ancestors, this notch held a spiracle , an unnecessary structure in an animal without an aquatic larval stage. There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular
2160-415: The embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for
2220-414: The exception of Paleognathae and Anseriformes . The penis is an intromittent organ used to transfer sperm into the female genital tract (i.e., vagina or cloaca ) for potential fertilization and, in the case of placentals , also for the excretion of urine . The penises of different animal groups are not homologous with each other, but were created several times independently of each other in
2280-559: The female genital tract and deposits sperm". Pizzles are represented in heraldry , where the adjective pizzled (or vilené ) indicates that part of an animate charge 's anatomy, especially if coloured differently. Sex organ A sex organ , also known as a reproductive organ , is a part of an organism that is involved in sexual reproduction . Sex organs constitute the primary sex characteristics of an organism. Sex organs are responsible for producing and transporting gametes , as well as facilitating fertilization and supporting
2340-408: The female reproductive tract that make direct contact with male genitalia or male products (sperm, spermatophores ) during or immediately after copulation". It is hard to find a common origin for gonads . However, gonads most likely evolved independently several times. At first, testes and ovaries evolved due to natural selection . A consensus has emerged that sexual selection represents
2400-411: The gynosome a "female penis" and insisted to drop the definition of penis as "the male copulatory organ". Motivations for using the term "female penis" include that such a term "is easier to understand and much more eye-catching" and that the gynosome have "analogous features" with male penises. Meanwhile, critics have argued that it does not fit the intromittent organ definition of "a structure that enters
2460-515: The increase in size and yolk content of eggs may have permitted, and coevolved with, direct development of the embryo to a large size. Features of amniotes evolved for survival on land include a sturdy but porous leathery or hard eggshell and an allantois that facilitates respiration while providing a reservoir for disposal of wastes. Their kidneys (metanephros) and large intestines are also well-suited to water retention. Most mammals do not lay eggs, but corresponding structures develop inside
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2520-410: The latter, however, was limited by their position in the terrestrial food-chain , which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators. While
2580-684: The male and the ovary in the female are called the primary sex organs . All other sex-related organs are known as secondary sex organs . The outer parts are known as the genitals or external genitalia , visible at birth in both sexes, while the inner parts are referred to as internal genitalia , which in both sexes, are always hidden. In plants, male reproductive structures include stamens in flowering plants, which produce pollen . Female reproductive structures, such as pistils in flowering plants, produce ovules and receive pollen for fertilization. Mosses , ferns , and some similar plants have gametangia for reproductive organs, which are part of
2640-448: The name. A problem with this definition is that the trait ( apomorphy ) in question does not fossilize , and the status of fossil forms has to be inferred from other traits. Older classifications of the amniotes traditionally recognised three classes based on major traits and physiology : This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics , as
2700-408: The number of holes behind each eye. In anapsids , the ancestral condition, there are none; in synapsids (mammals and their extinct relatives) there is one; and in diapsids (including birds, crocodilians , squamates , and tuataras ), there are two. Turtles have secondarily lost their fenestrae, and were traditionally classified as anapsids because of this. Molecular testing firmly places them in
2760-454: The other planarian, giving and receiving sperm. In most plant species, an individual has both male and female sex organs (a hermaphrodite ). The life cycle of land plants involves alternation of generations between a sporophyte and a haploid gametophyte. The gametophyte produces sperm or egg cells by mitosis . The sporophyte produces spores by meiosis , which in turn develop into gametophytes. Any sex organs that are produced by
2820-462: The other tetrapods somewhere during Romer's gap , when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic). The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of
2880-540: The penis was referred to as a "yard". The Oxford English Dictionary cites an example of the word yard used in this sense from 1379, and notes that in his Physical Dictionary of 1684, Steven Blankaart defined the word penis as "the Yard, made up of two nervous Bodies, the Channel, Nut, Skin, and Fore-skin, etc." According to Wiktionary , this term meant (among other senses) "rod" or "bar". As with nearly any aspect of
2940-406: The penis. A bone called the baculum is present in most placentals but absent in humans, cattle and horses. In mammals, the penis is divided into three parts: The internal structures of the penis consist mainly of cavernous, erectile tissue , which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae). Canine penises have a structure at the base called
3000-619: The penis. The external genital organs appeared in the Devonian , about 410 million years ago, when tetrapods began to abandon the aquatic environment. In fact, the necessity to overcome the absence of a liquid phase in which to release the gametes was achieved through the transition to internal fertilization . Among amniotes, the development of an erectile penis occurred independently for mammals , squamates ( lizards and snakes ), testudines (turtles), and archosaurs ( crocodiles and birds ). Over time, birds have lost this organ, with
3060-406: The plant will develop on the gametophyte. The seed plants , which include conifers and flowering plants , have small gametophytes that develop inside the pollen grains (male) and the ovule (female). In flowering plants, the flowers contain the sex organs. Sexual reproduction in flowering plants involves the union of the male and female germ cells, sperm and egg cells respectively. Pollen
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#17327809518383120-457: The presence of an astragalus for better extremity range of motion , the diminished role of skin breathing , and the complete loss of metamorphosis , gills , and lateral lines . The presence of an amniotic buffer, of a water-impermeable skin , and of a robust, air-breathing, respiratory system , allow amniotes to live on land as true terrestrial animals . Amniotes have the ability to procreate without water bodies . Because
3180-506: The rest of the reproductive system, whether internal or external. The Latin term genitalia , sometimes anglicized as genitals , is used to describe the externally visible sex organs. In general zoology , given the great variety in organs, physiologies, and behaviors involved in copulation , male genitalia are more strictly defined as "all male structures that are inserted in the female or that hold her near her gonopore during sperm transfer"; female genitalia are defined as "those parts of
3240-457: The scrotum; the protuberance grows larger and straighter to form the penis; the inner urogenital swellings grow, wrap around the penis, and fuse in the midline to form the penile raphe . Each organ/body part in one sex has a homologous counterpart. The process of sexual differentiation includes the development of secondary sexual characteristics , such as patterns of pubic and facial hair and female breasts that emerge at puberty. Because of
3300-427: The sex organs are indistinguishable from each other but, in other cases, male and female sex organs are clearly different. Similar gametangia that are similar are known as isogametangia. While male and female gametangia are known as heterogametangia, which occurs in the majority of fungi. Amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota , a large group that comprises
3360-729: The size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood. A similar pattern can be seen in modern amphibians. Frogs that have evolved terrestrial reproduction and direct development have both smaller adults and fewer and larger eggs compared to their relatives that still reproduce in water. Fish and amphibian eggs have only one inner membrane,
3420-401: The soft shell. Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating . Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst
3480-447: The strong sexual selection affecting the structure and function of genitalia, they form an organ system that evolves rapidly. A great variety of genital form and function may therefore be found among animals. In many other vertebrates , a single posterior orifice (the cloaca ) serves as the only opening for the reproductive, digestive, and urinary tracts (if present) in both sexes. All amphibians , birds, reptiles , some fish, and
3540-399: The urethra. Male and female genitals have many nerve endings, resulting in pleasurable and highly sensitive touch. In most human societies, particularly in conservative ones, exposure of the genitals is considered a public indecency . In humans, sex organs/genitalia include: External Internal External Internal In typical prenatal development , sex organs originate from
3600-567: The vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the Carboniferous period . Those of Amniota are defined as the smallest crown clade containing humans , the Greek tortoise , and the Nile crocodile . Amniotes are distinguished from the other living tetrapod clade — the non-amniote lissamphibians ( frogs / toads , salamanders , newts and caecilians ) — by
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